1. Field of the Invention
The present invention generally relates to a metal vapor discharge lamp. More specifically, the present invention is directed to an alkali metal vapor discharge lamp having a ceramics outer envelope operated in the pulsed mode, and to an ignition method for such an alkali metal vapor discharge lamp.
2. Description of the Related Art
A pulsed alkali metal vapor discharge lamp containing cesium is widely used as an infrared radiation lamp in a technical field such as infrared imaging utilizing infrared radiation. For instance, lamps having sapphire envelopes using cesium as a light emission metal, of a power range of several kilowatts (KW) are reviewed in a conference preprint entitled "Design of Pulsed Alkali Vapor Lamps Utilizing Alumina, Yttria and Sapphire Envelopes" presented by William R. Campbell at Illuminating Engineering Society (IES) in 1971. Such a pulsed alkali metal vapor discharge lamp normally maintains a low-power discharge (i.e., a simmer operating mode) to an extent such that the extinction of arc discharge does not occur, while discharging with a high-power pulse (i.e., a pulse operating mode) when ignition is required. Such light emission in the pulse operating mode is utilized for various applications.
FIG. 1 is a front view of the above-mentioned conventional infrared radiation lamp 100. The infrared radiation lamp 100 includes an inner envelope 1 made of ceramics, electrodes 2A and 2B formed of a tungsten coil, caps 3A and 3B mounted at opposite ends of the inner envelope 1, which caps are made of, for instance, niobium, metal conducting members 4A and 4B, external conducting members 8A and 8B, an outer envelope (jacket) 10 made of quartz, and end caps 11A and 11B for sealing opposite ends of the outer envelope 10 and connecting the metal conducting members 4A and 4B with the external conducting members 8A and 8B. The end caps are made of metal to be easily bonded to glass, such as Kovar (tradename), or Fernico (tradename). The inner envelope 1 is filled with cesium under a vapor pressure of 0.5 atm (380 Torr), mercury and rare gas. The interior of the outer envelope 10 is maintained under a vacuum, or inert atmosphere. Cesium contained in the inner envelope 1 has a resonance line at a wavelength near 850-900 nm. When discharge is conducted in the inner envelope 1, infrared radiation is realized by self-absorption of the resonance line and line spectrum of infrared.
The inner envelope 1 of another conventional infrared radiation lamp as shown in FIG. 1 is filled with cesium, mercury and rare gas. The rare gas is sealed under the pressure of normally 20 Torr, and the interior of the outer envelope 10 is maintained under a vacuum, or inert atmosphere.
In the above-mentioned conventional infrared radiation lamp 100, it has been required to increase the infrared radiation efficiency. However, the following three problems on an improvement of the infrared radiation efficiency still remain.
(1) In the conventional pulsed alkali metal vapor discharge lamp enclosing cesium, the reactivity between cesium and the inner envelope material is still indefinite.
(2) Details of the infrared radiation mechanism are not yet analyzed.
(3) Accordingly, concrete measure for increasing the infrared radiation efficiency have not been investigated, and a sufficiently high efficiency of the infrared radiation has not yet been established.
The present invention has been achieved so as to solve these conventional problems, and has a primary object of the present invention to provide a pulsed alkali metal vapor discharge lamp having a higher infrared radiation efficiency and a longer life.
Another object of the present invention is to provide an ignition method which makes easy the ignition of the above-mentioned pulsed alkali metal vapor discharge lamp and extends a life of the discharge lamp.
Still another object of the present invention is to provide a cooling system for the above-identified alkali metal vapor discharge lamp.